The in vitro slice technique was used to study neuronal activity in human cortical tissue removed during neurosurgical procedures for intractable epilepsy and in monkey neocortex rendered epileptogenic by injection of alumina gel. In both cases, biopsies were guided by electrocorticographic signs of epileptiform activity. Intracellular recordings were made from 167 neurons in human tissue and from 73 neurons in monkey tissue samples. There was little spontaneous activity in these biopsies and no indication of spontaneous cellular bursting. Stimulation at the pial surface or in white matter evoked synaptically driven activity that was primarily excitatory. Graded bursts of activity could be elicited from some cells, but no all-or-none paroxysmal depolarization shifts were recorded. Inhibitory postsynaptic potentials were relatively rare. Intracellular injections of dye in a small number of neurons revealed no obvious differences between bursting and nonbursting neurons. These in vitro studies of chronic epileptic cortex have thus far provided few clear insights into the basic mechanisms of epilepsy.